queue collector exports on disk and retry without a time limit

docs/scheduler/observability.md

@@ -64,6 +64,18 @@ ordinary DNS.
 runs on the same host as the agents and restarts on failure. Telemetry isn't
 the control plane, so degraded telemetry isn't degraded operation.
 
+Both collector tiers queue outgoing batches on disk (`file_storage`) and retry
+a failing batch until the receiving end accepts it, so a restart or a short
+outage of the next hop delays telemetry instead of dropping it. The queue holds
+at most 5000 batches. Two limits remain:
+
+- A full queue makes the exporter refuse new batches and log the drop. The
+  journald receiver has already moved its cursor past a refused batch, so an
+  outage longer than the queue can hold still loses data, later and a bounded
+  amount.
+- Retry has no time limit, so the exporter keeps resending a batch that the
+  store rejects with a retryable error, and enough of those fill the queue.
+
 ### Why two tiers
 
 **Agents export only to the hive tier.** Exporting straight to `upstream.endpoint` would mean